The murine leukemias, which closely resemble the disease in man, provide an excellent model system for the study of both the disease process and those cell virus interactions which elicit the leukemogenic response. Using this system we are studying the immunological, virological, biochemical, and ultrastructural events which occur between virus inoculation and the development of generalized leukemia. Since an understanding of the mechanisms involved in the spontaneous regression of neoplasma offers potential insight into both the etiology and the effective control of neoplastic disease, a significant proportion of our effort has been directed toward the characterization of a reproducible, virus induced, spontaneously regressing leukemia developed in our laboratory. Current emphasis is on the characterization of both host and virus factors which are responsible for the regression process. Studies initiated earlier on vertical transmission of murine leukemia, the role of DNA dependent RNA polymerase in leukemogenesis, and the characterization of kinetics of leukemia virus synthesis are continuing. Our pathogenesis studies on virus induced neoplasms are being extended to include virus induced murine mammary carcinoma.